Durability of Fence Posts

Bob Batson rcb at kc.rr.com
Tue Nov 21 16:56:46 EST 2000

                         Durability of Fence Posts

These days, farmers, gardeners, landscapers, and others who require
fenceposts (and other outdoor wooden structures in contact with the
ground) with a long useful life often use treated wood, in part
because it is difficult to find reliable information about the
durability of non-treated wood. For environmentally concerned
growers, the long-term fence post trials conducted at Oregon State
University's Forest Research Laboratory, which began in 1928, provide
a rare source of objective data on the durability of non-treated
posts of various timber species (and steel). And for those willing to
use treated posts, the trials provide comparative data for various
types of preservative treatments.

The most recent report on the OSU fence post trials is _Service Life
of Treated and Untreated Fence Posts: 1996 Post Farm Report_ by
Jeffrey J. Morrell, Donald J. Miller, and Philip F. Schneider
(Research Contribution 26, August 1999, available upon request from
Forestry Publications Office, Oregon State University, Corvallis, OR
97331-7401.). Below, we abstract from this report, with a focus on
the durability data for untreated fence posts.

The OSU post farm, site of all trials, is in the Peavy Arboretum, near
Corvallis. This site has silty-clay loam soil with pH 5.4 and 4.7%
organic matter in its upper layers. Climate at the site is mild, which
"favors growth of wood-destroying organisms throughout the year."
Annual precipitation is 41 inches, on average. Generalization of the
results obtained at this site to other locations must be done with
caution, but we suspect that data on RELATIVE durability of untreated
posts would be valid in a wide variety of situations. However, some
preservatives work better in dry climates than in wet climates. Note
that the post farm's posts are not stapled or nailed, and they are not
subject to stresses common in actual fencing.

In general, posts are most susceptible to deterioration near the
ground line. At the OSU post farm, fungi alone and fungi combined with
termites have caused most failures; termites alone, as well as other
insects (mainly carpenter ants and woodboring beetles) have caused
relatively few failures. The prime temperature range for fungal decay
is 75 degrees F to 90 degrees F.

At the post farm, annual inspections (in October) consist of pulling
laterally on the tops of the posts and examining failures to determine
their causes. On the basis of such inspections over a period of many
years, the following table shows the average service lifetimes of
untreated posts.

                                   (%)       AT GROUNDLINE    SERVICE
                                                (INCHES)   LIFE (YEARS)
Alder, red      split              25            19.7           5
                 round, peeled     100            11.8           3
Ash, Oregon     split              30            18.7           6
Buckthorn,      round, peeled      70             8.9           5
  cascara        round, unpeeled    35            16.7           8
  Alaska-        split, one tree    --            17.7          19
  Incense-       split               0            19.7          14
  Port-Orford-   split               0            23.6          20
  Redcedar,      split, dark         0            19.7          24
   western       split, dark         0            18.7          22
Cottonwood,     split              20            21.7           5
  black          round, unpeeled    95            13.8           4
Cypress,        round, unpeeled   100            12.8           4
Douglas-fir     round, unpeeled    60            18.7           7
                 square              0            15.7           6
                 square              0            15.7           4
                 round, unpeeled    48            13.8           7
                 square              5            14.8           4
                 round, four        80            15.7           4
                  strips peeled
Fir, grand      split              65            21.7           9
  Mountain       square, dry        --            14.8           3
  Western        square              0            15.7           6
Larch,          square              0            15.7           7
Madrone,        round and split    40            20.7           6
Maple,          split              25            19.7           7
Oak, Oregon     split              20            18.7          18
  Lodgepole      round, peeled,     55            15.7           5
                  dead trees
                 round, peeled,     55            15.7           4
                  live trees
                 round, four        80            11.8           3
                  strips peeled
  Ponderosa      square              0            15.7           6
  Sugar          square              0            15.7           7
  Idaho white    square              0            15.7           6
Redwood         square              0            15.7          21
Spruce, Sitka   square              0            15.7           6
Tanoak          round, unpeeled   100            11.8           4
Yew, Pacific    round, peeled      10            15.7          25

And here is information on groups of untreated wood and steel posts at
the post farm, some of which had not failed as of 1996:

Of 25 western juniper posts, 14 split and 11 round, with 40% sapwood
and groundline perimeters of 22.6 inches, 16% remained after 66
years, with an average life of 23 years for those that failed. The
tops of remaining posts showed slight to severe decay.

Of 22 black locust posts, 14 split and eight round, with 20% sapwood
and groundline perimeters of 13.8 inches, 18% remained after 61 years,
with an average life of ll year for those that failed. The tops of
remaining posts showed no to slight decay.

Of 26 osage-orange posts, 15 split and ll round, with l0% sapwood and
groundline perimeters of 18.7 inches, l00% remained after 66 years.
The tops of remaining posts showed no to slight decay.

Of 25 steel L-section posts with aluminum paint, 88% remained after
48 years, with an average life of 48 years for those that failed.

Of 25 steel T-section posts with red oxide paint, 64% remained after
48 years, with an average life of 36 years for those that failed.

Of nine steel I-section posts with green enamel paint, 100% remained
after 48 years.

Of 10 steel U-section posts with green enamel paint, 78% remained
after 48 years, with an average life of 33 years for those that

Of 10 steel T-section posts with green enamel paint, 100% remained
after 48 years.
Reprinted with permission from the October 2000 _HortIdeas_. Copyright
2000 by Greg and Pat Williams. _HortIdeas_ (ISSN 0742-8219) is published
monthly by Gregory and Patricia Y. Williams, 750 Black Lick Road,
Switch, KY 40328 U.S.A. Annual subscription rates: U.S., $20 periodicals
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_HortIdeas_ is: gwill at mis.net. _HortIdeas_ is now on the world wide web
at http://www.users.mis.net/~gwill/hi-index.html.


Bob Batson                            L 39 12 14 N 94 33 16 W
rcb at kc.rr.com                         Kansas City
TCS - Mystic Fire Priest              USDA Zone 5
Under the most controlled conditions, the experimental apparatus
will do exactly as it pleases.

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